Modern communications networks have been constructed to supply and share almost incomprehensible amounts of information. Regardless of the current capabilities of communications systems, there is a demand for more information and communication services. This demand could be satisfied using additional signal carriers, i.e. optical fibers. Each additional carrier adds to the space required for forming connections between optical fibers and related optoelectronic devices. This is contrary to the prevailing trend for succeeding generations of signal carriers to occupy less space than current systems.
The manufacturing process for optical fibers requires the fabrication of a pre-form, including a wave-guide of higher refractive index surrounded by a silica cladding of lower refractive index. Heating the perform in a furnace raises its temperature to cause melting and subsequent drawing down of an optical fiber from the molten pre-form. Generally the glass fiber includes one or more polymer coatings, also known as buffer coatings, applied in-line to the outside diameter, or cladding, of the optical fiber. A coated optical fiber, known also as a GGP fiber, includes a permanent polymer (P-coat) applied over the silica cladding before application of additional buffer coats as discussed previously.
U.S. Pat. No. 5,381,504 and U.S. Pat. No. Re. 36,146 describe commonly used, commercially available GGP fibers (Glass, Glass, Polymer) that include a doped silica core, a silica cladding, and a permanent polymeric coating or P-coat encircling the cladding. The dimensions of the layers of material comprising a commercially available GGP fiber produce an accumulated fiber diameter of approximately 250 microns to which a silica core and a reduced silica cladding contribute 100 microns. Addition of a P-coat increases the diameter to about 125 microns. After application of two standard buffer coats (the first to provide microbend protection, the second to provide abrasion resistance), the coated optical fiber reaches the final diameter of approximately 250 microns. Other types of fiber, referred to as standard fibers, do not include a permanent polymer layer, i.e. they are non-GGP fibers. Common non-GGP fibers include a silica core and cladding for a combined diameter of 125 microns and two standard buffer coats to give a final, coated optical fiber diameter of approximately 250 microns. The inner or primary buffer typically has a lower Shore D hardness than the outer or secondary buffer.
U.S. Pat. No. 5,644,670 describes the formation of a bare optical fiber including an optical fiber core, cladding and polymer covering. Repeated statements indicate that the bare optical fiber preferably has a diameter less than 128 microns and the polymer covering has a Shore hardness of D55 or more. Constraints on the diameter of a bare fiber relate to the need to mate with optical fiber connectors that typically have cross sectional dimensions of 125 microns. The use of a polymer coating with a high Shore hardness prevents damage to the cladding of a bare optical fiber by crimp connectors. Attachment of a crimp connector, to the terminal portion of a bare optical fiber, first requires removal of protective primary and secondary coatings from a jacketed broad bandwidth optical fiber cable that uses the bare optical fiber as the wave-guide element. A similar protected 125 micron wave-guide element is described in European Patent Application EP0953857 A1. In this case an optical fiber ribbon comprises a plurality of optical fibers arranged in a row. Each optical fiber comprises a core, a cladding and a non-strippable thin coating made of a synthetic resin, from 2 microns to 15 microns thick, coated around the cladding. The non-strippable thin coating provides protection to the underlying cladding and glass core during fiber connection to an optical fiber connector or planar lightwave circuit (PLC). An optical fiber ribbon includes a primary coating having a thickness up to twice the diameter of an optical fiber and a secondary coating that may be from 20 microns to 100 microns thick. U.S. Pat. No. 5,644,670 and EP0953857 A1 both address the effect of thickness of a polymer covering or non-strippable coating on the incidence of damage to the cladding layer while clamping a crimp connector on the end of an optical fiber from which primary and secondary coatings have been removed. There is no information related to the overall mechanical strength of bare optical fibers.
In many optoelectronic devices and optical fiber containment structures, there is limited space to accommodate large diameter fibers that include primary and secondary coated layers, as described above. This has produced a desire for finer optical fibers, preferably capable of withstanding bending around small radii for a range of communications applications. Communication applications, as indicated, represent optical fiber networks that need interconnection using standard optical fiber connectors. A standard connector typically includes a 125 micron ferrule, into which an optical fiber is inserted to be securely held in alignment either with another optical fiber or a related optoelectronic device. Standard coated optical fibers typically have a diameter of about 250 microns, as discussed previously. Inter-fiber connections can only be made with these fibers after removing coating to provide a stripped fiber having a diameter small enough for insertion into a standard ferrule. Fiber damage commonly occurs during the stripping process to remove buffer coating from an optical fiber. Such damage could be avoided through use of a strong optical fiber that did not require stripping of buffer coats to fit snugly in standard 125 micron ferruled connectors. A further advantage would be for the fiber to show no sign of wear or degradation by abrasion or chemical attack.
The P-coat in a GGP fiber typically comprises an epoxy resin that may be cationically curable, preferably by exposure to a suitable form of actinic radiation. Other known cationically curable resins include cycloaliphatic epoxy groups or vinyl ethers in their structure. Manufacture of a GGP coated optical fiber requires solidifying of the silica clad fiber as it is drawn from the furnace followed immediately by application of the P-coat. A P-coat typically contains an iodonium salt as a cationic photoinitiator that interacts with suitable radiation to cure the polymer. One or more protective buffer coats, applied over the cured P-coat, provide protection and abrasion resistance while raising the overall diameter of the GGP fiber construction to approximately 250 microns.
The following discussion refers to other cationic photoinitiators, most of which have been used in applications unrelated to coated optical fiber production. U.S. Pat. Nos. 5,340,898, 5,468,902, 5,550,265 and 5,668,192 discuss various iodonium borates and organometallic borates as photoinitiators. These patents, however, do not mention borate anion containing photoinitiators as curatives for polymeric materials applied to optical fibers. U.S. Pat. No. 6,011,180 discloses organoboron photoinitiators suitable for photopolymerization of monomer compositions having acid group functionality. The photoinitiators are described by the generic formula G+ (R)4B− wherein G+ includes onium cations, particularly sulfonium cations or iodonium cations and (R)4 represents substituted alkyl and aryl groups.
Several references describe photoinitiators including “polyborate” anions, exemplified by EP 775706, U.S. Pat. No. 5,807,905 and WO 9852952. European patent EP 834492 describes polyiodonium cation-containing photoinitiators without mentioning application of these materials to coating optical fibers.
U.S. Pat. No. 4,655,545 discloses a glass fiber for fiber optic transmission networks. The reference discusses optical fibers extrusion coated with a fluorine containing resin. It is known that fluorine containing resin coatings cause reduction of the mechanical strength of optical fibers compared with similar optical fibers coated using resins that contain no fluorine. The reference attributes the lowering of mechanical strength to the generation of fluorine gas or hydrogen fluoride at the time of melt extrusion. According to the reference, these acidic gases pass through a first baked layer and reach the glass surface to weaken the glass fibers by erosion of the glass or interference with chemical bonding between the baked extruded coating and the glass surface. U.S. Pat. No. 5,181,269 presents a contrary finding by suggesting optical fiber strength improvement using acidic cationically photocured coatings containing hydrolyzable components e.g. hexafluoroarsenate and hexafluorophosphate anions. Although including materials and coating methods, this patent (U.S. Pat. No. 5,181,269) provides no supportive data related to optical fiber strength.
U.S. Pat. No. 5,554,664 describes energy activated salts with fluorocarbon anions. The reference discusses the advantage of catalysts with non-hydrolyzable anions for adhesives and related coatings used in electronics applications. Hydrolyzable anions exemplified by hexafluorophosphate (PF6−) and hexafluoroantimonate (SbF6−) ions react in the presence of moisture to produce corrosive hydrofluoric acid. The reference otherwise addresses onium salts containing methide and imide anions and gives examples of borate anion initiators.
The mechanical properties and lifetime of a coated optical fiber may be adversely affected by inappropriate selection of a photoinitiator to cure polymeric fiber coatings. The previous discussion of fluorinated photoinitiators describes formation of hydrofluoric acid in the presence of moisture. In the presence of corrosive materials, such as hydrofluoric acid, some GGP fibers exhibit a decrease in fiber strength, as shown in dynamic fatigue tests, when they are placed in a high temperature/high humidity environment. Unfortunately, high temperature and high humidity conditions are relatively common during operation of vehicles including submarines and related naval craft, space craft, aircraft, and other applications that currently use, or have the potential to use, optical fibers and related devices. In these cases, it is important to avoid corrosive materials.
In view of the desire to increase the volume of information transmitted via communication networks, using signal carriers having greater strength retention, there is a need for a small diameter, coated optical fiber that retains its strength following exposure to high temperature and high humidity. Small diameter coated optical fibers offer the possibility for producing fiber optic devices containing more signal carrier connections than current devices using fibers coated to a diameter of 250 microns.